In a statement marking the anniversary of the murder Séamus Dooley, Irish Secretary called on the British and Irish governments to appoint a panel of independent, international experts to carry out an investigation into the murder of Martin and into the failed police investigation which has been mired in controversy.

He said the failure of the investigation and the suspicion that those linked to the murder were protected because of potential links to the police “cast a long shadow over the criminal justice system and policing in Northern Ireland”.

The union is to write to Prime Minister Teresa May and Taoiseach Leo Varadkar seeking support for an international inquiry.

Séamus Dooley added: “Today our thoughts are with Martin’s family, colleagues, comrades and friends. Sixteen years after his brutal murder the killers of Martin O’Hagan have not been brought to justice. It has long been suspected that those who ordered his killing were paid police informants linked to the Loyalist Volunteer Force (LVF).

“We need an independent, international panel to investigate the murder and the failed police investigation. The O’Hagan murder and subsequent investigation raises fundamental questions about the relationship between the police and Loyalist paramilitaries. It is not sufficient to dismiss this killing as an unsolved murder. We need to know why the murder remains unsolved and has effectively been deemed unsolvable.”

He added: “We had a review by the Police Ombudsman of Northern Ireland in 2013 but we cannot accept that the file should be closed.

“This is a murky affair which demands a new investigation by an external panel looking afresh at all aspects of the murder and the subsequent investigation, with particular reference to links to police informers.

“This is not the only unsolved murder in Northern Ireland but Martin O’Hagan was the only journalist killed during the Troubles because of his journalism. He was killed because of his relentless pursuit of the truth and in his memory the NUJ continues to demand the truth about his murder.

“No barriers, financial or political, should prevent an independent, international investigation into the murder of Martin O’Hagan. Fear of the uncomfortable truths which may emerge is no justification for not asking difficult questions.”